Induction of toxin-specific neutralizing immunity by molecularly uniform rice-based oral cholera toxin B subunit vaccine without plant-associated sugar modification

Plants have been used as expression systems for a number of vaccines. However, the expression of vaccines in plants sometimes results in unexpected modification of the vaccines by N-terminal blocking and sugar-chain attachment. Although MucoRice-CTB was thought to be the first cold-chain-free and unpurified oral vaccine, the molecular heterogeneity of MucoRice-CTB, together with plant-based sugar modifications of the CTB protein, has made it difficult to assess immunological activity of vaccine and yield from rice seed. Using a T-DNA vector driven by a prolamin promoter and a signal peptide added to an overexpression vaccine cassette, we established MucoRice-CTB/Q as a new generation oral cholera vaccine for humans use. We confirmed that MucoRice-CTB/Q produces a single CTB monomer with an Asn to Gln substitution at the 4th glycosylation position. The complete amino acid sequence of MucoRice-CTB/Q was determined by MS/MS analysis and the exact amount of expressed CTB was determined by SDS-PAGE densitometric analysis to be an average of 2.35 mg of CTB/g of seed. To compare the immunogenicity of MucoRice-CTB/Q, which has no plant-based glycosylation modifications, with that of the original MucoRice-CTB/N, which is modified with a plant N-glycan, we orally immunized mice and macaques with the two preparations. Similar levels of CTB-specific systemic IgG and mucosal IgA antibodies with toxin-neutralizing activity were induced in mice and macaques orally immunized with MucoRice-CTB/Q or MucoRice-CTB/N. These results show that the molecular uniformed MucoRice-CTB/Q vaccine without plant N-glycan has potential as a safe and efficacious oral vaccine candidate for human use.